1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus having a deflection scanning exposure unit such as a polygonal mirror for deflecting light.
2. Description of the Related Art
In the field of electrophotographic image forming apparatuses, a tandem type image forming apparatus has been known having a plurality of image forming sections, in which different color images are sequentially transferred on a recording material held on a conveying belt for speeding up.
FIGS. 21A and 21B show an example of the tandem-type color image forming apparatus. FIG. 21A is a general schematic view. This color image forming apparatus includes a transfer material cassette (not shown) mounted on the body bottom. The transfer materials placed on the transfer material cassette are taken one by one and supplied to the image forming section. In the image forming section, a transfer conveying belt 10 is flatly stretched along a plurality of rollers in a conveying direction of a transfer material and is driven by a drive motor 21 for conveying the transfer material. The transfer material is electrostatically attracted on the transfer conveying belt 10 by applying a bias on an absorption roller (not shown) arranged on the surface of the transfer conveying belt 10 on the most upstream side of the transfer conveying belt 10. Four photosensitive drums 14 are linearly arranged to oppose the belt conveyance surface as drum-type image bearing members. A developing unit that is the image forming section includes the photosensitive drum 14, each toner (not shown) for colors of C (cyan), Y (yellow) M (magenta), and K (black), a charger (not shown), and a developer (not shown). Within a casing of each developing unit, a predetermined space is provided between the charger and the developer, and through this space, the circumferential surface (the image bearing member surface) of the photosensitive drum 14 is exposed by an exposure unit 8 composed of at least one laser scanner.
FIG. 21B is a drawing showing the exposure unit 8 in detail. Referring to FIG. 21B, a divergent light beam (laser beam) emitted from a light source 1 is substantially collimated with a collimator lens 2; an aperture stop 3 limits the passing light beam (light quantity); a cylindrical lens (cylinder lens) 4 having predetermined refracting power in a sub-scanning direction focuses the light beam that has passed through the aperture stop 3 on a deflection surface 5a of a light deflector 5 (described below) within a sub-scanning section as a substantially linear image; a polygon mirror (rotatable polygonal mirror) 5 for deflecting light as a deflecting element is rotated at a predetermined speed in arrow A direction in the drawing by a drive unit (not shown) such as a motor; an optical element 6 having fθ characteristics is composed of a refraction unit and a diffraction unit; the refraction unit is formed of a single plastic toric lens 6-a having power different in a main scanning direction from that in a sub-scanning direction, and both lens surfaces of the toric lens 6-a in the main scanning direction are aspheric; the diffraction unit is formed of a long diffraction optical element 6-b having power different in a main scanning direction from that in a sub-scanning direction; and a beam detection sensor (BD sensor) 7 is arranged outside an image region for determining the writing timing in the main scanning direction. By writing images after a predetermined period of time since receiving a signal from the BD sensor 7, the process can be synchronized in the main scanning direction.
Each charger (not shown) uniformly charges the circumferential surface (image bearing member surface) of the corresponding photosensitive drum 14 with predetermined electric charge, and the exposure unit 8 exposes the charged circumferential surface of the photosensitive drum 14 (the image bearing member) in accordance with image information so as to form electrostatic latent images. Then, the developer (not shown) produces (develops) toner images by transferring toner to a low-potential portion of the electrostatic latent images. A transferring material (not shown) is positioned with the conveyance surface of the transfer conveying belt 10 therebetween. The toner images formed (developed) on the circumferential surface (image bearing member surface) of each corresponding photosensitive drum 14 are attracted and transferred onto the surface of the transfer material by an electric charge in the conveyed transfer material produced by the transfer electric field in the corresponding transfer material (not shown). The toner images transferred on the transfer material are thermally fixed on the sheet in a fixing unit (not shown) including a pressure roller and a heating roller, and the transfer material with the fixed toner images is discharged outside the apparatus. The transfer conveying belt 10 may also be an intermediate transfer belt, on which each toner for colors of C (cyan), Y (yellow) M (magenta), and K (black) is once transferred, and then is secondarily transferred on the transfer material. A tandem-type color printer includes the exposure unit 8 and the developing unit (not shown) for each toner for colors of C (cyan), Y (yellow) M (magenta), and K (black). Therefore, for executing main scanning magnification adjustment, main scanning writing position adjustment, and sub-scanning writing position adjustment, a patch (not shown) is depicted, and registration adjustment is performed based on patch information.
The conventional image forming apparatus described above experiences an unevenness of exposure due to the displacement between beams in a plurality of laser beams, polygonal axis tangle in the deflection scanning exposure unit, and sub-scanning exposure displacement followed by the polygonal face tangle. A primary reason for the exposure unevenness is micro-displacement of the beam for each polygonal face along the sub-scanning direction from the ideal sub-scanning exposure position. The exposure unevenness is accompanied by sinewave color density unevenness (due to the polygonal axis tangle) having a period corresponding to one rotation of the polygonal mirror, random color density unevenness due to the polygonal face tangle, and color density unevenness having a period corresponding to the number and relative displacement of the beams for each of the colors. The exposure unevenness is also accompanied by complicated color density unevenness due to a periodic beat. When there are two frequencies, for example, the periodic beat is formed of small fluctuations with a period of the difference between the two frequencies.
Conventionally, in order to suppress the sub-scanning displacement amount, a two-beam laser or four-beam laser has been used for a plurality of laser beams, or a plurality of laser units have been assembled with fine positional adjustment for eliminating the displacement amount. Also, in order to suppress the exposure unevenness due to the polygonal axis tangle in the deflection scanning exposure unit and the sub-scanning exposure displacement followed by the polygonal face tangle, a method for suppressing the sub-scanning exposure displacement amount has been employed by strictly controlling accuracy rating in polygonal axis tangle and polygonal face.
In such a manner, for speeding up the process and improving image quality, the accuracy rating requirement has a strong tendency to become very strict for the displacement between beams in a plurality of laser beams, polygonal axis tangle, and polygonal face tangle. Moreover, in the conventional method of strictly controlling accuracy rating in the displacement between beams in a plurality of laser beams, polygonal axis tangle, and polygonal face tangle, a problem arises in that productivity cannot be increased.
In Japanese Patent Laid-Open No. 04-200065 (described above), it is proposed that when a scanning pitch d in the sub-scanning direction is large, the exposure amount owing to semiconductor laser is increased, and when the scanning pitch d is small, the exposure amount owing to the semiconductor laser is decreased. The exposure amount per unit area is thereby maintained within a specified tolerance.
However, according to the exposure amount correction method of Japanese Patent Laid-Open No. 04-200065, with increasing change in sub-scanning displacement, higher performance is demanded of a semiconductor unit. For example, output characteristics and higher resolution are required along a wide operating range of the semiconductor unit, resulting in higher cost for the semiconductor unit.
Also, according to the exposure amount correction method of Japanese Patent Laid-Open No. 04-200065, although the method is functioning to suppress the unevenness of color density effectively to some extent, it has a problem of accuracy. The reason of the accuracy problem in Japanese Patent Laid-Open No. 04-200065 is specifically described below.
FIGS. 23A and 23B are drawings of dose distributions of laser light viewed in the sub-scanning direction; FIG. 23A shows the distributions when the resolution is 600 dpi; and FIG. 23B shows the distributions when the resolution is 2400 dpi. Referring to FIGS. 23A and 23B, an exposure spot of a laser beam on the drum surface is approximated by a Gaussian distribution, and the spot diameter in the sub-scanning direction (the size of a spot with the light quantity equaling or exceeding (1/e2)×the light quantity at the center of the spot) is 70 μm in FIG. 23A and is 50 μm in FIG. 23B. In the resolution 600 dpi of FIG. 23A, the sub-scanning line interval is 42.3 μm, which is comparatively large relative to the spot diameter, so that the bottom of the Gaussian distribution is contained roughly within a range bounded by one neighboring pixel along sub-scanning direction. Whereas, in the resolution 2400 dpi of FIG. 23B, the sub-scanning line interval is 10.6 μm, which is comparatively small relative to the spot diameter, so that the bottom of the Gaussian distribution is spread across four neighboring pixels. Thus, it is necessary to consider from one pixel to three pixels in the front and the rear of the noticed pixel in the sub-scanning direction. If the exposure amount is decreased when the pitch d between pixels is small, its influence extends across four neighboring pixels, so that the exposure amount is relatively decreased over the range of the four neighboring pixels. Conversely, if the exposure amount is increased when the pitch d between pixels is large, its influence extends across four neighboring pixels, so that the exposure amount is relatively increased over the range of the four neighboring pixels. In such a manner, when the pixels stand relatively close to each other in the sub-scanning direction, the exposure amount cannot be established only by the relationship with the adjacent pixel, so that it is understood that the correction control for maintaining the exposure amount per unit area constant is complex. In the relationship between the spot diameter and the resolution, if the spot diameter is larger than SQRT (2) (square root of 2) times the sub-scanning line interval (the scanning pitch d in the sub-scanning direction), the effect of the bottom of two neighboring pixels must be taken into consideration. If the resolution is improved without decreasing the spot diameter, other nearby pixels have a more pronounced impact and so must be taken into consideration.